The present invention relates to wall shear stress sensors. In particular, it relates to a fiber optic wall shear stress sensor.
A significant fraction of the total resistance to motion on airplanes and ships is due to surface friction, or skin friction. Measuring and determining this friction is complex and further complicated by the limited availability of skin friction gages. This is in contrast to the availability of gages for pressure, temperature and strain. Early work in this area involved obtaining data in pipe flows, where the wall shear can be directly related to the pressure drop, which is relatively simple to measure accurately. By measuring the small frictional force exerted on a movable element of the surface, one is able to obtain direct measurements of wall shear stress. These types of measurements work well for laminar, transitional and turbulent flows without prior knowledge of the state of the flow.
A great number of techniques for the measurement of wall shear stress have been devised over the years, ranging from inferring the skin friction from measuring the boundary layer profile or using some correlation or analogy to the direct measurement of the force on a surface. Although all of these techniques can be shown to work for some flow regime, indirect methods, such as Stanton tubes, Preston tubes, and surface hot-wire techniques to name a few, have not been shown to be reliable for complex flows such as 3D and/or unsteady cases with rough wall, curved walls, flows with injection or suction, or high-speed flows, especially those with high enthalpies, combustion and/or impinging shocks. Alternatively, direct measurements do not require any foreknowledge of the flow or its properties and can provide accurate results all the regimes mentioned above.
Direct measurements, refers to techniques that separate a small element, referred to as a floating head, from the wall and measures the tangential force that the flow imparts on it. Direct measurements are the most believable of all the techniques. The sensor is measuring the actual shear on the surface, without respect to the fluid, the state of the boundary layer, or chemical reactions. Since the floating head is level with the wall, the measurement is non-intrusive to the flow. The forces are very small, sometimes requiring large floating heads and expensive instrumentation to obtain accurate results. The direct measurement technique generally falls into two categories, nulling and non-nulling.
In the nulling design, called this because the floating head is returned to its null (or original) position, the floating head is acted upon by shear, but the sensor provides a restoring force to the head to keep it in place. The magnitude of that restoring force is monitored, which is equal to the shear force acting on the floating head. In one example, the floating head was supported by a beam which pivoted around a spring near the base. The movement of the head was sensed by a pair of capacitance plates, with an electromagnet used to supply the restoring force. Other nulling designs have required mechanical linkages and motors to return the head to the null position. Although the nulling design does remove some of the possible error introduced by the movement of the head, the sensors are very complex, mechanically unreliable, and have a slow time response, on the order of seconds.
In a non-nulling design, the floating head is allowed to deflect under the shearing load. This type of design is much simpler than a nulling design, allowing the removal of the restoring force mechanism and thus making much smaller time responses possible. However, this arrangement does allow the structure of the sensor to flex and this variation may introduce spurious results.
Another type of non-nulling gage is a design where the floating head is supported by tethers around its periphery. This design is sensitive to normal pressure changes because the tethers are more sensitive to motion normal to the wall than parallel to it. In macro-designs of the tether sensor, where the displacement was measured by strain gages mounted on the tethers, the sensor was not only sensitive to normal pressures, but also to temperature changes as the strain gages were very near the flow. In supersonic flows, wall temperature can change significantly during a test. However, the heat flux boundary conditions are significantly different at the floating head than the rest of the wall, leading to significant temperature differences and, therefore, significant errors in the shear measurement. In micromachined designs, the strain gages have been replaced by capacitance or by an external laser/photoiode system. However, the drawbacks of the table-top design remain, limiting the use of the sensor to simple flows without large temperature or pressure variation.
Another non-nulling design is a cantilever beam concept. In this design, the floating head is attached to a single cantilever beam which flexes with the application of shear to the head. The displacement is measured by the strain caused by the displacement at the base of the beam. If the displacements are kept to a minimum, the issue of head protrusion into the flow is negligible. This design offers high stiffness for normal forces, while being relatively weak for tangential forces, providing a sensor that is insensitive to normal pressure variations. The concept is very simple, rugged and has a small time response. If the sensing head is prepared from similar materials as the surrounding wall, errors from temperature mismatches will be minimized. Also, the concept can be easily extended to measure in two directions, removing the directional ambiguity of other designs.
Winter (xe2x80x9cAn Outline of the Techniques Available for the Measurement of Skin Friction in Turbulent Boundary Layers,xe2x80x9d Prog. Aerospace Sci., 1977, Vol. 18, pp. 1-57) was able to use resistance strain gages in a large balance, which allowed sensitive measurements in a static environment. Schetz and Nerney (xe2x80x9cTurbulent Boundary Layer with Injection and Surface Roughness,xe2x80x9d ATAA Journal, 1977, Vol. 15, No. 9, pp. 1288-1294) made the next step to semi-conductor strain gages, improving the output 100 fold. A series of skin friction sensors based on strain gages mounted on the cantilever beam in a non-nulling arrangement have also been produced that are useful for measuring subsonic or cool supersonic flows. However, as the Mach number range increases, hot flows are encountered. In addition, the study of supersonic flows with injection of combustible gases such as hydrogen also necessitated a new design of skin friction gages. High enthalpy flows are simulated using an object. During this simulation, the skin friction gage encounters both high temperatures and high electromagnetic field environments.
Acharya et al. (xe2x80x9cDevelopment of a Floating Element for the Measurement of Surface Shear Stress,xe2x80x9d AIAA Journal, March 1985, Vol. 23, No. 1, pp. 410-415) describe a floating-element instrument having a central component which is a tension galvanometer which supports the element itself. The position of a target on the back of the element is determined using a fiber-optic scanner. A reflector is lined-up perfectly with the fiber. The reflector is only partially reflective, and therefore, only part of the light is returned through the fiber. The returning light is shined on a photodiode and transformed to voltage. The nulling part of the circuit moves the head and the reflector back to the same voltage, which is the same location on the reflector. This design is extremely complicated requiring a feedback mechanism and an actuator to rezero the head. The alignment of the fiber and the reflector must be precise, making construction of the device difficult. Use of the partial reflection reflector limits the temperature that the sensor can be used. Since all of the electronics are internal, the sensor is large, complex and temperature sensitive. Moreover, this design can only be used for nulling applications.
By the present invention, the sensing technology has moved to an interferometric use of fiber optics, allowing a design principle based on deflection instead of strain. It has been discovered that an interferometric use of fiber optics not only offer increased sensitivity to displacement but also new design concepts that increase sensitivity to shear. This permits the area of the floating head and the length of the cantilever beam to be decreased and, therefore, the whole sensor package may be miniaturized.
Fiber optic sensors provide a means for overcoming the temperature and electromagnetic field sensitivity issues. In particular, optical fiber sensors: 1) have an inherent immunity to electromagnetic interference; 2) are capable of avoiding ground loops; 3) are capable of responding to a wide variety of measurands; 4) have excellent resolution; 5) are capable of avoiding sparks; and 6) can operate at temperatures ranging from 800xc2x0 C. to 1900xc2x0 C. Murphy et al. (U.S. Pat. No. 5,301,001) describe a fiber optic strain sensor based on the extrinsic Fabry-Perot interferometer (EFPI). This sensor is a displacement sensor wherein a single mode silica fiber transmits light from a laser diode to the sensor element. The sensor comprises a single mode fiber, used as an input/output fiber, and a multimode fiber, used purely as a reflector, to form an air gap within a silica tube that acts as a Fizeau cavity. The single mode fiber transmits light from a laser diode to the sensor element. At the opposite end of the input fiber, the laser light signal is partially reflected and partially transmitted across the gap separating the ends of the input fiber and the multimode fiber. The reference signal and the reflected signal interfere and propagate back through the input fiber to a photodiode detector. Changes in the separation distance between the surfaces of the fibers produce a modulation of the output signal current. The strain sensor is used to measure small displacements by attaching the sensor to the surface of the material to be measured. As the strain of the material is transferred to the sensor, the separation distance between the surfaces of the fibers changes and the resulting displacement is measured. The strain is inferred from the resulting displacement. Although previous wall shear stress sensors had a strain measurement to infer the shear stress, the EFPI sensor as configured for strain measurements proved to be ineffective for the stated purpose. Wall shear stress measurements necessarily take place in fluid flows which have static pressure different than that of the atmosphere at sea level. The EFPI as configured for strain measurements is slightly pressure sensitive, but when compared to the magnitude of the strain in a conventionally-configured wall shear stress sensor caused in a normal fluid flow, this sensitivity is large and ruins the measurement. Applicants discovered that it was no longer necessary to rely on a strain measurement, and that the wall shear stress sensor should be configured completely around a displacement measurement. This removed the pressure sensitivity and increased the overall sensitivity of the sensor.
An object of the present invention is to provide a fiber optic wall shear stress sensor based on deflection type measurements.
Another object of the present invention is to provide a fiber optic wall shear stress sensor that is capable of deflecting due to a shear force.
Another object of the present invention is to provide a fiber optic wall shear stress sensor that is less sensitive to temperature changes than previous sensor designs.
These and other objects of the invention were achieved by the present invention which is a fiber optic wall shear stress sensor. The sensor comprises a floating head supported by a physical arrangement and at least one optical fiber positioned in an operable relationship to the floating head. An interferometric region is formed between the floating head and each optical fiber. The interferometric region changes in response to a shear force on the floating head.
In another embodiment of the invention, the fiber optic wall shear stress sensor comprised a floating head supported by a physical arrangement. A reflector is positioned in an operable relationship to the floating head such that the reflector moves in response to the floating head. At least one optical fiber is positioned in an operable relationship to the reflector. An interferometric region is formed between each optical fiber and the reflector. The interferometric region changes in response to a shear force on the floating head.
When measuring wall shear stress with the fiber optic sensor of the present invention, the sensor is provided. Light is directed through the optical fiber. The fiber optic wall shear stress sensor is exposed to a fluid flow such that the floating head is deflected. This deflection causes a change in the interferometric region and light is reflected back through the optical fiber. If the arrangement with the reflector is being used, the light is reflected from the reflector back through the optical fiber. The change in the interferometric region is measured and correlated to the a wall shear stress.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be obtained by means of instrumentalities in combinations particularly pointed out in the appended claims.